Hydrogen in its many compound forms is an essential part of life on earth: it combines with oxygen to form water (H2O) and is present in virtually every life molecule; it also combines with carbon to form hydrocarbons, the fuels that cover most of our current energy needs.

Pure hydrogen, in the form of its two atom molecule H 2 , is a fuel itself and its energy density per unit of mass is higher than for any other substance. In this respect, it is important to notice that hydrogen is not an energy source but an energy carrier, linking primary energy sources to energy services such as light, heat and transport. Like electricity, another energy carrier, hydrogen is not readily available and must be produced using a different form of energy.

It is interesting to note that, until the industrial revolution, the main source of energy for most people was wood – a hydrocarbon fuel with a carbon to hydrogen atoms ratio (C/H) of 9:1. Improved technology led to widespread the use of coal (C/H of 1.5:1) then oil (C/H of 1:2) and natural gas (C/H of 1:4): it is clear that there is trend where carbon-rich fuels are progressively substituted by hydrogen-rich ones; a simple extrapolation of such trend would lead to a scenario in which pure hydrogen is the fuel of choice.

Drivers for the adoption of hydrogen as a fuel

Up until the last fifty years, the main drivers in energy usage patterns have been price and availability of fuels; environmental issues, in particular pollution ones, have been of minor significance.

It is only within the last decades that the threat of climate change resulting from increasing concentrations of greenhouse gases in the atmosphere has been identified and understood; since then, reductions in greenhouse gas emissions from energy applications has become a major issue for the energy sector.

More recently, concerns about the security of current energy sources have also increased, thus bringing energy security up in the energy policy agenda.

Benefits of hydrogen as a fuel

A key benefit of hydrogen as a fuel is its versatility: it can in fact be derived from a variety of sources: from hydrocarbons, such as natural gas, biomass and coal, and from electrolysis of water, using electricity generated from renewable, nuclear or fossil sources. Research is also being carried out on new production routes, for example using hydrogen-producing bacteria, or splitting water using high temperature heat from solar or nuclear energy. Such versatility could help increase the security of energy systems through diversification. In addition, some of the routes, such as those based on wind and biomass energy, are intrinsically more secure than those based on fossil fuels, in terms of both long-term availability and geographical distribution of the sources.

Once produced, hydrogen can then be used to carry the energy to where it is needed; hydrogen can then be combusted in a fuel cell or an engine to provide an energy service, such as transportation, heat or useful power. It is also important to note that hydrogen can be stored more easily than electricity and that its combustion produces only water, thus avoiding any form of pollution at the point of use.

As regards greenhouse gas emissions, it is expected that within perhaps two decades several routes to hydrogen could help achieve material CO2 reductions at competitive prices. In this respect, hydrogen from non-fossil energy sources such as renewable electricity, biomass and electricity from nuclear appears to have the best performance. Also, hydrogen derived from fossil sources such as natural gas or coal could bring material contributions, but only if coupled with reliable and cheap technologies to capture and store the CO2 resulting from hydrogen production.